1. Field of the Invention
This invention relates to the processing of semiconductive substrates to form integrated circuit (IC) chips. More particularly, this invention relates to techniques for accurately aligning the substrates with the processing equipment.
2. Description of the Prior Art
Prior art processes use optically-recognizable registration marks formed on semiconductive wafer substrates to effect alignment of the substrates with the processing equipment, such as wafer steppers. Commonly, a so-called registration mask was used at the beginning of the process to form one or more marks on each wafer. Difficulty was frequently encountered, however, because the optical appearance of the mark became altered by the epitaxial layer subsequently grown as part of the IC processing. Such alteration (sometimes referred to as "epitaxial wash-out") obscured the mark, or distorted its apparent position. In any event, the result was increased number of IC chip rejects.